It is necessary to protect an electrical cable where it passes through an opening in a wall of a metallic housing to prevent a possible sharp edge of the metal defining the opening from abrading the outer protective sheath of the cable or even severing the insulation about an electrical conductor thereby possibly causing an electrical short. One way to provide this protection is to line the opening with a resilient insulative grommet. However, such a grommet does not hold the cable. Thus, if a pulling force is applied to the cable, this force is resisted by the electrical conductors of the cable where they are attached to terminals inside of the housing. The result could be that the electrical conductors become disconnected, possibly resulting in a short or making the metal housing live.
Often strain relief clamps are provided to hold the cable where it passes through the wall. The clamp functions to resist any pulling forces so that they are not reflected at the electrical terminations. Such a strain relief clamp could include a U-shaped receiver defining a cavity for receiving the cable and an insert for closing the cavity and pushing the cable towards the base of the receiver to effect bending of the cable. When the strain relief clamp is inserted into the opening, the cable is firmly held and the clamp not only resists pulling of the cable but also is locked against longitudinal movement with respect to the housing wall which holds the clamp.
Such a clamp, however, does not prevent rotation of the clamp in the circular opening in which the clamp is located. The wall opening must be substantially circular because it must be able to accommodate the passage of a pipe as some electrical codes require the wiring to be in a conduit. An example of a known two-piece strain-relief clamp is part No. 7 K -2 made by the Heyco Manufacturing Company. Unrestricted rotation of the clamp can result in the portion of the cable disposed inside of the housing becoming twisted which could again result in various forces being applied to the terminations of the electrical conductors which could cause, in an extreme case, disconnection of the conductors from the terminals. In certain applications, such as a enclosure for an electric garage door opener motor, where there is vibration attendant the operation of the equipment, Canada and various European countries are requiring that the cable is held from rotation with respect to the housing.